Preface: We all know that if Man of Steel bombs, there is no Justice League. This plan is contingent of the fact it will be a success.
To the point - Right after Man of Steel, announce 2 movies:
1. Superman/Batman: World's Finest
2. Wonder Woman
Immediately, you introduce the trinity. We all know the importance of these three characters in the DC universe. We can use this to popularize the main characters across a wide range of people while honoring their valued lore.
The Superman/Batman: World's Finest movie will do three important things - it reintroduce Batman to the unified DC Universe (recast), it brings back the familiar face of Superman after the succes of Man of Steel, and it shows the audience how Superman's and Batman's worlds can intertwine and blend together. We don't need another origin story for Batman. We've had him for the past 8 years and we know who he is. It will a lot for people to swallow; going straight from the Dark Knight trilogy into the Justice League movie. This movie will serve as a smooth transition into a combined universe.
We need to be familiar with a third character before going into the Justice League. It will be too many characters to handle and no one will get their origins the attention they deserve. Wonder Woman is a great character to introduce for two reasons - she builds the female audience and has a rich history that can mesh with Superman and Batman. Finally, if she the actress doesn't gain much favor in the theaters, you can pull a Hulk - recast before the next movie.
Regardless, this got to be much longer than I wanted but that's my idea. I think its a quick and simple solution to build up to Justice League.
What do you guys think?